Exporting overwrite behaviour

[mtorpey]

Abd mentioned today that he created a bunch of feedback with FeedbackHelper, then exported it and did some post-processing in another application. He then exported grades and feedback from FeedbackHelper again (only meaning to export the grades) which overwrote the files and lost him some time.

Some thoughts here:

1. Knee-jerk reaction is to revert A unified export #23 and go back to separate grade and feedback export. But this isn't what most people would want to do, and has its own downsides.
2. Alternatively, we could automatically back up previous exported files when we overwrite. There may be other cases when this would be valuable. Even using moveToTrash on existing files would be a quick win.
3. Bigger picture: the concept of a walled-off fht format that can't be edited in other apps and can only "export" to plain text is questionable. As discussed in Load assignment from exported files #64, maybe the plaintext files should be the save format, and this way users could fearlessly edit an assignment in another application and see the changes on reloading in FeedbackHelper. Abd's issue is another hint that this might be better.

[mtorpey]

The last release uses “move to trash” on supported platforms.

[mtorpey]

This was resolved at least in some sense, and the more ambitious suggestion (#64) is tracked elsewhere.